Article http://pubs.acs.org/journal/aesccq
Interface Structure between Vitrinite and Inertinite from Shenmu Coal during Pyrolysis Wei Zhao,*,†,‡ Fusheng Yang,† Zhiyuan Yang,†,‡ and Anning Zhou*,†,‡ †
College of Chemistry and Chemical Engineering, Xi’an University of Science and Technology, 710054 Xi’an, Shaanxi, China Ministry of Land and Resources, Key Laboratory of Coal Resources Exploration and Comprehensive Utilization, 710021 Xi’an, Shaanxi, China
‡
ABSTRACT: This research investigated the pyrolytic characteristics of the interface between vitrinite and inertinite, exploring the structure and changes at the interface. The various features of macerals and their interface during pyrolysis were analyzed using Fourier transform infrared microspectroscopy and a scanning electron microscope, and the crystallite structures of the macerals were observed using a transmission electron microscope. The results showed an interaction between Shenmu vitrinite and inertinite at their interface. The interface generally showed a gradual changing of pyrolytic degree from the surface toward the center of the layers during pyrolysis. However, cracks from the escape of vitrinite volatile substances terminated at the inertinite layer, which then changed the original mode of heat transfer at the interface and caused a remarkable mutation. The compatibility of the interface structure between vitrinite and inertinite could be improved by heat treatment, which strengthens the aromatic skeletal structures. In addition, the growth of aromatic fringes could be enhanced but inertinite showed a faster progress of graphitization than vitrinite during pyrolysis. KEYWORDS: Coal maceral, vitrinite, inertinite, interface, structure, pyrolysis, micro infrared, thermal stability
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INTRODUCTION According to coal petrology, organic macerals of coal can be divided into vitrinite, inertinite, and liptinite. Different structures and properties of the three macerals lead to different applications in coal processing and utilization.1−4 Recent research has been conducted on the structural characteristics and chemical reactivity of macerals utilizing advanced analytical techniques, such as Fourier transform infrared spectroscopy (FTIR), NMR, X-ray photoelectron spectroscopy, X-ray diffraction, thermogravimetry coupled with differential scanning calorimetry, and gas chromatography mass spectrometry. These studies reveal that macerals show a diminishing trend in volatile yield and the content of hydrogen, olefins, and alkanes: liptinite > vitrinite > inertinite.5−8 Conversely, the content of aromatic carbon decreases in the order of inertinite > vitrinite > liptinite.9−11 Vitrinite has a higher oxygen content, containing numerous long aliphatic chains and bridges and a greater number of substituents on aromatic rings, whereas inertinite has many more oxygen functional groups, containing relatively higher aromaticity.12−15 CHar/CHal ratio does not change with carbon content in liptinite but it increases in vitrinite and inertinite.16 Coal pyrolysis is the foundation in coal processing and conversion. It is generally thought that the study of the pyrolytic features of coal macerals should provide helpful information about the behavior and regulation of coal pyrolysis © 2017 American Chemical Society
on a microlevel. Therefore, more researchers focus on the pyrolytic characteristics of coal macerals. Li,17,18 Strugnell,19 Cai,20 Megaritis,21 Iglesias,22 Das,23,24 Kawashima,14 Sun,25 Xie,26 Zhao,27 Malumbazo,28 and Roberts29 investigated the pyrolysis behavior and properties in the product of coal macerals. These studies demonstrated that the pyrolysis reactivity of the individual macerals are different and show a diminishing trend: liptinite > vitrinite > inertinite. Liptinite has the highest reactivity of devolatilization, while inertinite shows the highest characteristic temperatures of pyrolysis. Feng30 and Jin31 came to the same conclusions from liquefaction of macerals. Stanger32 and Xie33 used coal maceral concentrates to study the association between physical and chemical with thermal changes during pyrolysis, finding that tar release and gas evolution were associated with a rapid swelling event, which was substantially greater for the high vitrinite sample. Malumbazo34 investigated the impact of coal macerals and particle size on char formation in a packed bed combustion unit and found that the char formation and its reactivity was controlled more by the content of total inert maceral particles than by coal particle size. Received: Revised: Accepted: Published: 179
January 20, 2017 April 26, 2017 May 16, 2017 May 18, 2017 DOI: 10.1021/acsearthspacechem.7b00002 ACS Earth Space Chem. 2017, 1, 179−186
Article
ACS Earth and Space Chemistry Most previous research has focused on macerals themselves. However, many of these studies fail to study the interface of different macerals, especially the structure and property of the interface during thermal reaction. The structure of the interface often differs greatly from that of the interior of the macerals, which can result in different physical changes. Moreover, disaggregation and separation of coal macerals is by no means an easy task. Studies have shown that the particle size of coal must be crushed to less than 10 μm to ensure the dissociation of each maceral.35−37 In the routine use of coal, macerals mostly exist in coal particles in a combined or embedded form rather than a dissociated form. Therefore, the interface transformation of macerals usually occurs in coal conversion. The study of the interface structure of macerals during pyrolysis can explain the interface transformation during the heat treatment of coal. Also, it can also guide coal pyrolysis and utilization. This paper mainly focuses on two major macerals (vitrinite and inertinite) of the low metamorphic bituminous coal of northwestern China. A comparative study on the pyrolysis features and solid phase products of vitrinite and inertinite was conducted with a focus on the pyrolysis features at their interface. An effective method was established to study coal pyrolysis and its interface structure.
Table 1. Analysis of Raw Coal and Its Macerals sample
SMR
Proximate Analysis moisture (wt %, ad) 7.1 ash (wt %, db) 3.1 volatile matter (wt %, daf) 38.5 fixed carbon (wt %, daf) 61.5 Ultimate Analysis (wt %, daf) C 78.8 H 4.9 N 1.1 O 14.6 S (wt %, td) 0.5 Maceral Composition (vol %) vitrinite 64.5 inertinite 31.8 exinite 0.8 mineral 2.9
SMV
SMI
9.9 2.6 40.3 59.7
7.2 4.0 33.0 67.0
79.0 5.3 1.0 14.4 0.3
83.0 4.0 0.6 11.9 0.5
97.3 1.1 0.4 1.2
2.1 95.6 2.3
respectively. The samples were taken out after natural cooling to room temperature. Products Analyses. The infrared spectrum analysis of coal samples before and after pyrolysis was conducted by Nicolet iN10MX micro infrared imaging spectrometer with an auxiliary iZ10 FTIR optical detector connected. The infrared detection of MCT detector ranged from 4000−400 cm−1 and the signalto-noise ratio was greater than 25000:1 with a Ge crystal ATR objective. Changes of the surface topography of the coal samples before and after pyrolysis was observed by JSM-6460LV tungsten filament scanning electron microscopy (JEOL Ltd.), whose secondary electron resolution was greater than 3 nm. The magnification rate ranged from 5−300,000× and the imaging mode was secondary electron. High-resolution transmission electron micrographs of the surface of coal samples before and after pyrolysis were taken by Talos TEM from FEI Company. The point resolution rate was more than 0.12 nm. Accelerating voltage ranged from 20−200 kV. Magnification rate was 25−1,000,000×. Also, the STEM had a resolution of 0.16 nm and magnification of 150−250×.
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EXPERIMENTAL SECTION Coal Samples. Coal samples were 3−1 fresh coal collected from the Zhang Jiamao coal mine of Shenmu, Shaanxi Province. Shenmu vitrinite (SMV) and Shenmu inertinite (SMI) were stripped from raw coal (SMR) by hand picking and density gradient centrifugation. Their particle size was less than 0.125 mm after the disc mill grinding. The lump coal samples were prepared according to the “Preparation of optical slice of directed lump coal” from GB/T 16773-2008.38 Coal samples with clear and uniform layered distribution were sliced and polished. For the purpose of removing potential impurities affecting the subsequent experimental analysis, no adhesive reinforcement was used during the process of slicing, and additional abrasives were also avoided in the process of grinding and polishing. The lump coal samples were put into a 105 °C vacuum drying oven for 24 h. Proximate, ultimate, and maceral composition analyses of coal samples are shown in Table 1. Assignment and area of FTIR peaks for SMV and SMI are shown in Table 2. Also, the structural parameters of SMV and SMI are displayed in Table 3. Experimental Procedure. The pyrolysis of SMV and SMI were carried out in a thermal analysis system of TGA/DSC1 HT from Mettler Toledo Company, Switzerland. The working temperature ranged from room temperature to 900 °C at the heating rate of 10 °C/min. The inert atmosphere was highly purified nitrogen. A polarization microscope of German-made Leica DM4500P was used to observe oil-immersed lump coal samples (eyepiece magnification was 10× and objective lens of immersion oil magnification was 50×). A suitable maceral form and position was chosen and marked. Then, the pyrolytic experiment of the lump coal samples was conducted, utilizing a self-made horizontal pyrolysis furnace. The samples were put into a quartz tube and pushed into the constant temperature zone of the pyrolysis furnace. Nitrogen gas, the shielding gas in the experiment, was aerated in advance for 15 min to replace the air in the quartz tube. Then the quartz tube was heated at a heating rate of 5 °C/min to 350, 450, 550, and 650 °C for 30 min,
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RESULTS AND DISCUSSIONS TG/DTG Analysis. The thermal stability of SMV and SMI was examined by TG/DTG and the results are shown in Figure 1. As seen in Figure 1, thermal weight loss of SMV is greater than SMI, indicating that vitrinite more easily undergoes thermal decomposition than inertinite. When the temperature is below 400 °C, there is relatively little weight loss for either SMV or SMI. This is the stage in which coal softens and degases and gas and tar are separated out, but the decomposition temperature of SMI is higher than SMV. When the temperature is 400−700 °C, a larger weight loss peak appears with the highest peak around 450 °C. The weight loss rate of SMV is obviously higher than SMI, showing a lower thermal stability in vitrinite and a lower pyrolysis reactivity in inertinite. This is mainly due to the higher hydrogen content, lower aromaticity, more aliphatic chains and bridges, and the alkyl side chain being present in vitrinite. Conversely, inertinite contains more C−C bonds and condensed branched chains in aromatic rings. The bond energy of hydrocarbon in aliphatics is lower than that in aromatics; this make the vitrinite easier to be 180
DOI: 10.1021/acsearthspacechem.7b00002 ACS Earth Space Chem. 2017, 1, 179−186
Article
ACS Earth and Space Chemistry Table 2. Assignment and Area of FTIR Peak for SMV and SMI wavenumber/cm−1
3600−3100
3100−3000
3000−2900
2880−2800
1790−1680
1680−1520
1520−1400
1200−950
950−700
assignment of spectral peak
−OH −NH
−CH(Ar)
−CH3 −CH2
CO −COOH
CC(Ar) CO
−CH3 −CH2
CO Ar−O−Ar
C−H(Ar)
sample
31.26 19.86
0.38 1.92
−CH3 −CH2 −CH 7.84 4.63
1.75 1.61
15.09 18.62
12.67 13.23
4.09 2.33
9.31 8.19
8.41 10.62
SMV SMI
Table 3. Structural Parameters of SMV and SMIa,b C
H
O
H/C
O/C
fa
2(R − 1) C
R
6.58 6.92
5.30 4.00
0.90 0.74
0.805 0.578
0.137 0.107
0.731 0.781
0.464 0.641
2.527 3.218
structural paramenters sample a
fa =
SMV SMI
(100 − Vdaf ) × 1200 b fa 1240 × Cdaf
(
=2 1−
R−1 C
) − HC Microinfrared Analysis. Each lump coal sample was cut and polished along its layered vertical distribution surface. Its organic macerals and the condition of the integrated components at the interface were observed under a microscope with results displayed in Figure 2. As shown in Figure 2, the composition of coal macerals in Zhang Jiamao mine is less complicated but the amount of interlayering is greater. The vitrinite is rich in desmocollinite and contains a large amount of endogenous pore. The inertinite content is higher and mostly half fusinite. Except for its layered features, some of the inertinite is mingled with the vitrinite. To investigate the structural features of coal vitrinite, inertinite, and their combinational interface during pyrolysis, a cutting plane of coal block was selected (as shown in Figure 2). Suitable maceral forms and positions were chosen and marked, as shown in Figure 3. Then, the pyrolysis experiments were carried out. The marking locations on the cutting plane after pyrolysis were found using an IR microscope, and the infrared analysis was completed by attenuated total reflection (ATR). Because of severe cracks that occurred at the interface under the pyrolysis temperature of 650 °C, it was difficult to identify and find the right combinational interface. Therefore,
Figure 1. TG/DTG curves of vitrinite and inertinite during pyrolysis.
pyrolyzed. It also reflects that during the main pyrolysis stage the chemical reaction of vitrinite focuses on the cleavage of side chains and bridges among aliphatic hydrocarbon of organic macromolecular form as well as of inertinite derives from the breaking of bridges and branched chains among aromatic hydrocarbons.23,25,27
Figure 2. Layered distribution of coal macerals, immersion oil and emitted light at 500×. (a) Vitrinite layer (degradinite) and inertinite layer (semifusinite). (b) Inertinite layer (semifusinite) and vitrinite layer (degradinite, vitrinite is mixed with inertodetrinite). (c) Layer of vitrinite mixed with inertinite (central section: composed of degradinite, semifusinite, and macrinite). 181
DOI: 10.1021/acsearthspacechem.7b00002 ACS Earth Space Chem. 2017, 1, 179−186
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ACS Earth and Space Chemistry
Figure 3. FTIR spectrograms of SMV, SMI, and its interface with different pyrolysis temperature.
H2O from the oxygen-containing functional groups of low-rank coal escape faster and hardly produce carboxyl and methoxyl after 450 °C.23,25 The peak of −COOH and −CO from SMV disappears basically at 450 °C, while it is relatively sluggish for SMI. Infrared spectrum of the interface between SMV and SMI shows similar changes but the intensity is different. In order to further investigate the mutual interaction of macerals at the interface during pyrolysis, a difference spectrum is calculated between the spectra of the arithmetic mean of SMV and SMI with their interface. The results are shown in Figure 4.
the temperature gradient was limited to only reach a maximum of 550 °C. In Figure 3, SMV shows a stronger absorption peak at around 3100−3600 cm−1, demonstrating high content of OH, which is consistent with the higher H and O elements in the ultimate analysis as shown in Table 1. The small peak at 3044 cm−1 is the stretching vibration peak of aromatic −CH where SMV is slightly lower than SMI because of its lower content of aromatic hydrogen and less aromaticity. The peak at 2800− 2900 cm−1 is the stretching vibration peak of aliphatic −CH that is stronger in SMV than SMI. The peak at 700−900 cm−1 reflects the substitution of aromatic hydrogen. The absorption peak of SMI in this range is slightly stronger than SMV, showing a higher degree of aromatic hydrogen substitution, which is consistent with its higher degree of aromatization. The infrared spectra of coal samples under different pyrolytic temperature are alike, but the intensity of the absorption peaks differs. With the increase of pyrolytic temperature, the strong hydrogen bonds −OH of SMV and SMI stretching at 3100− 3600 cm−1 become weak, while aliphatic −CH stretching at 2800−2900 cm −1 shows an initial decrease, then an increase, but then decreases again at 450 °C. The reason is probably that the surface aliphatic chains of highly volatile Shenmu coal break at the beginning of the pyrolysis but with the increase of temperature aliphatic chains cyclize and functional groups condense greatly forcing the free hydroxyl to move to a low frequency area; the coal tar pitch and hydrocarbon generating from the rupturing of weak bonds also move to the surface. The trend of aromatic −CH stretching at 3044 cm−1 and aromatic skeletal vibration at 1390 cm−1 is in contrast to the aliphatic −CH, suggesting that the coal pyrolysis process results in the removal of aliphatic molecules and the relative increase of the basic structural unit.24,27 While the change of SMV and SMI is slightly different, aromatic −CH content has reached the maximum when SMI is pyrolyzed at 550 °C but SMV is still on the increase. The vibration peak of −COOH and −CO at 1700 cm−1 also displays the trend of increasing first and decreasing later. This is because at the beginning of pyrolysis, CO2 and
Figure 4. Difference spectra of arithmetic mean of SMV and SMI with their interface.
In Figure 4, significant differences can be observed between the infrared spectrum at the interface of SMV and SMI and their average spectrum, suggesting a difference of reaction between the interface and the macerals during pyrolysis. From the difference spectrum, peaks at 2910, 1610, and 1700 cm−1 are gradually strengthened with the increase of pyrolysis temperature, indicating that interactions probably exist in the side chains, oxygen-containing functional groups, and aromatic 182
DOI: 10.1021/acsearthspacechem.7b00002 ACS Earth Space Chem. 2017, 1, 179−186
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ACS Earth and Space Chemistry
Figure 5. IR distribution with pyrolysis depth of interface between SMV and SMI.
Figure 6. SEM images of coals during pyrolysis with different temperature.
increase, which indicates a remarkable change at the interface of vitrinite and inertinite. The absorption peak of aromatic rings at 1000−1300 cm−1 increases with temperature, suggesting macromolecular fragments are more likely to be generated at the interface of macerals during pyrolysis and to polymerize into aromatic structures. In order to investigate changes at the interface between SMV and SMI and with depth into the maceral layers of the coal samples during the heating process, semicoke block after pyrolysis at 450 °C was cut and scanned with the infrared spectral line from the surface to the center (distance: 5000−0). The results are shown in Figure 5. In Figure 5, the distribution curve shows infrared spectrum characteristics on the interface of SMV and SMI changing with depth. A gradual change can be observed in the infrared spectrum from the surface to the inside showing that the coal pyrolysis is a thermal diffusion process from the outside to the inside that is related to the traditional method of heat treatment and the diathermancy of coal. There is a certain temperature gradient between the surface and the inside of a coal sample during the heating process, leading to different degrees of pyrolysis. As shown in Figure 5, the pyrolytic degree was significantly higher in some inside areas of the interface as compared to other surrounding places, which is probably due to a variety of factors. First, in the coal samples there are a large number of pores and fractures that possibly collude with each other during pyrolysis because of the evolution of volatile substances and which changes the original mode of heat transfer. Second, the volatile content and thermal conductivity
hydrocarbons at the border of vitrinite and inertinite. The differences in low frequency areas also show a mutual interference of vitrinite and inertinite in aromatic skeletal structure and aromatic substitution during pyrolysis. Inertinite contains more oxygen-containing functional groups, while vitrinite has a higher content of hydrogen and oxygen. In the process of pyrolysis, vitrinite can help to provide inertinite with hydrogen, thus promoting the stabilization of radical fragments through achieving active hydrogen to accelerate the generation of low molecular fragments. Meanwhile, the nonactive oxygen of vitrinite participates in the reaction probably due to the induction of oxygen-containing functional groups in inertinite during pyrolysis. The vibration of covalent bonds in coal becomes more intense with the increase of heat during the thermal reaction. In addition, a part of the covalent bond dissociates to generate volatile matter, but the other part of the structure recombines before the fracture of the covalent bond. During the recombination, the covalent bond dissociates into high molecular weight coke and small molecular weight volatile matter.39 Vitrinite, which contains more side chains and branched chains, transforms from a smaller aromatic cyclic structure to a polycyclic aromatic hydrocarbon structure after being heated. Inertinite has a higher aromaticity and condensed degree, and aliphatic bridges exist among the aromatic rings.40 The side chains and branched chains from vitrinite and the aliphatic bridges among the aromatic rings from inertinite combine through the fracture and recombination of covalent bonds during heating. Figure 4 shows how the peaks at 1610 cm−1 and low frequency region in the difference spectra 183
DOI: 10.1021/acsearthspacechem.7b00002 ACS Earth Space Chem. 2017, 1, 179−186
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ACS Earth and Space Chemistry
Figure 7. TEM images of coals and semicokes.
is different between vitrinite and inertinite. Vitrinite with more volatile matter tends to form cracks in the process of pyrolysis, while inertinite is relatively stable. The cracks from vitrinite may terminate on the inertinite layer, thus causing an intensified pyrolysis reaction at this joint point. Moreover, uneven heating may also be the result of a variation in the type and quantity of minerals present in the coal. SEM Analysis. On the basis of the microspectroscopy, a scanning electron microscope was used to observe the deformation characteristics on the interface of SMV and SMI at different temperatures. The results are shown in Figure 6. As seen in Figure 6, cracks can be obviously observed inside inertinite at 350 °C but the form of vitrinite is unaltered. This may be due to the mass internal porosity of inertinite, which tends to absorb more gas (like CO2, H2O, and so forth).24,33 At temperatures below 350 °C, the effect of drying, degassing, and desorption of gas causes the formation of internal cracks in inertinite. These cracks tend to develop upward along the interface when encountering the dense structure of vitrinite. At 450 °C, internal cracks and holes appear in vitrinite, suggesting the increase of vitrinite pyrolysis. In the interlayers of inertinite there appears slight cracking and deformation, which may result from the difference of thermal expansion and cold contraction between vitrinite and inertinite. At 550 °C, both vitrinite and inertinite present obvious deformation. The cracks are developed inside each component and their interface performance is relatively minor, suggesting that the cracks do not develop upward along the interface as consensus believes. At 650 °C, the degree of pyrolysis deepens and so does the polycondensation of solid phase products. The weight loss and densification of semicoke leads to volume shrinkage,41,42
causing the connection of the internal pore in different components. TEM Analysis. A high-resolution transmission electron microscope (HRTEM) was used to observe the imaging of atomic planes in the macerals and their pyrolytic products. The TEM images of SMV, SMI, and semicokes at 650 °C are shown in Figure 7. An image processing technique was applied on the high-resolution images to produce the lattice fringes of the macerals or chars.43 A parallelogram catenation method proposed by Mathews et al. was used in this study to assign aromatic sizes to the extracted HRTEM fringes.44,45 All the lattice fringes
